Many electronic components, such as amplifiers for wireless communications receivers and transmitters, contain signal amplifiers to enhance the performance of the systems. These electronic components typically utilize a bias current source circuit to apply a bias or a gain to the signal.
Generally, the current source may be biased by a constant gain, a linear gain, or a linear-in-dB gain. A constant gain simply amplifies the current source by a constant gain. A linear gain is biased linearly as the received signal varies. A linear-in-dB gain applies an exponential amplifier gain in response to a linear change in the received signal.
For example, FIG. 1 is a plot that shows a linearly changing output current source with respect to the input voltage. The horizontal axis represents the input voltage Vin of the received signal, and the vertical axis represents the output current Iout. As the input voltage Vin increases, the output current Iout increases linearly with respect to the input voltage Vin.
FIG. 2 is a circuit diagram that illustrates a circuit 200 that linearly biases a current source with respect to an input voltage as illustrated in FIG. 1. The circuit 200 has a control amplifier 210, transistors M1, M2, and M3, and resistor R1. Control amplifier 210 has inputs Vin and a feedback line. The output of the control amplifier 210 is electrically coupled to the gate of transistor M3. The source of transistor M3 is electrically coupled to the feedback line of control amplifier 210 and resistor R1 to ground. The drain of transistor M3 is electrically coupled to Vdd through transistor M1. The gates of transistors M1 and M2 are electrically coupled to the drain of transistor M3. The drain of transistor M2 is electrically coupled to the output current Iout.
While this circuit clips the output current Iout at predetermined input voltages due to circuit limitations, the circuit illustrated in FIG. 2 does not have the ability to clip or limit the output current Iout at different desired levels. Furthermore, the circuit illustrated in FIG. 2 cannot provide a linear-in-dB current source with respect to the input voltage.
Many applications, however, would benefit from a linear-in-dB gain amplification or current clipping. For example, a power amplifier (PA) driver with built-in current steering variable gain amplifier (VGA) utilizes a dumping transistor to vary the output current as the power levels change. At maximum output power, the current in the dumping transistor is almost zero. However, when the output power is decreasing, the current in the dumping transistor increases until all the current is steered to the dumping transistor. Consequently, power is lost or wasted at low output power levels. Because a typical PA driver consumes a large portion of current consumption from a chip, it is desirable to reduce the amount of current that is wasted through the dumping transistor.
Therefore, there is a need to bias the current to the PA driver with the built-in current steering VGA scaled linearly-in-dB to a predetermined level when the output power of the PA driver is reduced. Furthermore, there is a need to generate a linear output current with respect to the input voltage with maximum and/or minimum clipping levels.